The present invention relates to body implantable devices, and more particularly to the prostheses incorporating the characteristics of stents and grafts, and having a flow dividing capability useful for implantation in branched blood vessels.
A variety of treatment and diagnostic procedures involve devices intraluminally implanted into the body of a patient. Among these devices are stents, such as disclosed in U.S. Pat. No. 4,655,771 (Wallsten). The Wallsten devices are tubular, braided structures formed of helically wound thread elements. The stents are deployed using a delivery catheter With the stent positioned at the intended treatment site, an outer tube of the delivery catheter is withdrawn, allowing the stent to radially expand into a substantially conforming surface contact with a blood vessel wall or other tissue.
Thread elements or strands formed of metal are generally favored for applications requiring flexibility and effective resistance to radial compression after implantation. Metal strands can be thermally formed by a moderately high temperature age-hardening process while mounted on a mandrel in the desired configuration. The strands, due to their high modules of elasticity, cooperate to provide the needed strength. Strand flexibility also permits a radial compression and axial elongation of the stent that facilitates intraluminal delivery of the stent to the treatment site. Because the self-expanding stent generally remains at least slightly radially compressed after fixation, its elastic restoring force can provide acute fixation.
The favorable combination of strength and flexibility is due largely to the properties of the strands after they have been age-hardened or otherwise thermally treated. The braid angle of the helical strands and the axial spacing between adjacent strands also influence strength and stability. Age-hardening processes are described in U.S. Pat. No. 5,628,787 (Mayer) and U.S. Pat. No. 5,645,559 (Hachtman, et al.).
An alternative stent construction features plastically deformable metallic structures, which also can be composed of helically wound metallic strands. Such stent does not require an outer tube or other feature to maintain it in a reduced-radius state during delivery. However, radial expansion requires a dilatation balloon or other expansion means.
Regardless of whether stents are self-expanding or plastically deformable, they characteristically have an open mesh or open frame construction, or otherwise are formed with multiple openings to facilitate radial enlargements, and to allow tissue ingrowth. Such stents typically expand axially or longitudinally as they radially contract, and in the case of resilient stents, also axially contract as they radially expand.
Prostheses with more tightly woven strands are known. For example, U.S. Pat. No. 4,681,110 (Wiktor) discloses a flexible tubular liner insertable into the aorta to treat an aneurysm. The liner is a tight weave of flexible plastic strands, designed to elastically expand against the aneurysm to direct blood flow past the aneurysm. The tight weave is intended to minimize leakage, so that the liner can effectively shunt blood through to eliminate the aneurysmal sac from the blood path.
The Wiktor device and others like it notwithstanding, those of skill in the art have continued to encounter difficulty in providing a device that simultaneously accommodates the competing needs of low permeability, strength and flexibility for radial compression and expansion. One known response to this difficulty is a combination stent graft, in which a compliant but substantially fixed-radius and tightly woven graft is sutured or otherwise coupled to a radially expandable stent. Upon release, the stent is intended to radially expand to the graft diameter. This requires a careful matching of the graft diameter with the lumen diameter at the treatment site. Otherwise, either an oversized graft is compressed between the stent and body tissue with undesirable folding or gathering of the graft material, or an undersized graft prevents the stent from radially expanding sufficiently to anchor the device.
Another difficulty arises from the fact that the stent layer and graft layer, even when they are constructed to undergo combined radial contraction and axial elongation, behave according to different relationships governing the amount of radial reduction for a given axial increase. When the stent framework elongates a greater amount for a given radial reduction, elongation of the composite structure tends to tear the bond joining the graft and the stent. Conversely, if the graft layer undergoes greater axial expansion, an unwanted increase in bending stiffness causes localized reductions in diameter when the stent graft is bent around tight radii. Negotiation through tortuous vascular passageways becomes more difficult, and in some instances impossible.
Several prosthesis constructions have been suggested for composite braided structures that combine different types of strands, e.g., multi filament yarns, monofilaments, fusible materials and collagens. Examples are found in International Patent Publications No. WO 91/10766, No. WO 92/16166, No. WO 94/06372, and No. WO 94/06373. A highly favorable combination of strength, resistance, range of treatable lumen diameters and low permeability has been achieved by woven composite devices featuring textile strands interbraided with either cold-worked or thermally set structural strands, as disclosed in U.S. patent applications Ser. No. 08/640,062 and Ser. No. 08/640,091, both filed Apr. 30, 1996 and assigned to the assignee of this application. Although such devices are well suited for a wide range of procedures, there are costs and complexities inherent in interweaving different types of strands.
The application of stent grafts to branched vessels is known, for example as disclosed in U.S. Pat. No. 5,522,880 (Barone, et al.), and U.S. Pat. No. 5,507,769 (Marin, et al.). The foregoing problems apply to stent grafts in general, and thus confront designers of bifurcated stent grafts as well.
Therefore, it is an object of the present invention to provide a bifurcated prosthesis structure that affords the advantages of stents and grafts, yet does not require an interbraiding of structural strands and textile strands.
Another object is to provide a process for manufacturing a bifurcated stent graft in a manner that better ensures that the open-frame structural layer and the low-permeability fabric layer remain securely fixed to one another as they undergo radial and axial enlargements and reductions.
A further object is to provide a bifurcated prosthesis that incorporates highly permeable open areas and low permeability covered areas, and thus is adapted for selective axial positioning to shunt blood flow where required while remaining open to branches of the vessel under treatment.
Yet another object is to provide a system, incorporating a bifurcated stent graft, that is less costly to manufacture and easier to implant.